The invention relates to a needle bar actuating device for double thread chain stitch and/or overlock sewing machines and has a primary object the provision of means for adapting the needle bar stroke length to the thickness of the fabric being sewn.
The problem is solved by means of a device for actuating the travel of the needle, provided with a kinematic assembly comprising a connecting-rod component rotatably connected to the needle rod, a crank component integral with one end of a rotating shaft, and a device for joining together the said connecting-rod and crank components, which device consists substantially of two pins which are arranged in succession and are integral with each other and the respective axes of which are arranged so as to be mutually offset and parallel, a first pin being rotatably engaged with the said connecting-rod component and a second pin being movably engageable, via locking members, with the said crank component in a plurality of positions which differ angularly from each other.
As is known, sewing machines of the said type have a special device for actuating the travel of the needle, which is made and designed in different ways depending on the specific requirements and technical abilities. An actuating device is in fact known, in which the needle rod is connected to an actuating lever oscillating angularly about a pin lying perpendicularly to the needle rod.
The actuating lever is located inside the arm of the sewing machine and is made to oscillate by appropriate kinematic mechanisms which extend inside the column of the sewing machine. An actuating device is also known, in which the needle rod is made to move in a vertical direction by a shaft passing through the arm of the sewing machine and oscillating angularly about its own axis, owing to the action of a drive shaft extending in the base of the sewing machine.
Finally, an actuating device is known, in which the needle rod is operated by a secondary shaft which has a continuous rotating motion generated by a main shaft located in the base of the sewing machine. The secondary shaft is located in the arm of the sewing machine and an appropriate kinematic assembly transforms the rotating movement of the secondary shaft itself into an alternating rectilinear movement of the needle rod.
All the abovementioned and currently known actuating devices have the characteristic feature that they practically cannot be modified without taking steps comparable to partial restructuring of the sewing machines provided with the said devices. For this reason, the travel of the needle in known sewing machines must be considered a non-modifiable characteristic feature of the machines themselves, which is preset once and for all and in a definitive manner during the design and construction stages.
However, it is known that in the case of sewing machines designed to produce, for example, a chain stitch using one needle and two threads, the needle travel cannot be constant and fixed in all situations, but, on the contrary, must be adapted to the type of sewing operation which is to be performed.
In fact, when fabrics of considerable thickness are sewn, a relatively long needle travel is required so that the needle itself can be inserted and then removed with certainty from the fabric (extension of the travel upwards). On the other hand, short travel is required when fast stitches must be made on thin fabrics. In other situations, medium-length travel may be appropriate.
It must also be pointed out that potentially long travel is required when a so-called bottom covering stitch is to be performed, i.e. a particularly complicated chainstitch requiring a considerable amount of thread.
The need to vary the needle travel in accordance with the type of stitch which is to be performed obviously conflicts with the already mentioned impossibility of adjusting the parts of a sewing machine so as to increase or reduce the needle travel. It follows that, in the past, when it was not possible to use a medium-length travel setting which was substantially acceptable for the operations in question, it was necessary to purchase various sewing machine models which differed from each other with respect to the extent of the needle travel.
This situation is negative since it adversely affects not only production costs, when there is the practical need to purchase several sewing machines which differ only with respect to the needle travel, but also the quality of work, when a less than adequate needle travel is accepted or in fact used, with a consequent excessive supply of thread and loose sitches or insufficient supply of thread and stitches which are too tight.
The technical task underlying the present invention is therefore to overcome the abovementioned drawbacks of the known art, designing a sewing machine provided with an actuating device which is able to vary the needle travel as required.
Within the scope of this technical task, it is an important object of the present invention to design a device which has a simple structure, can be easily applied to the existing sewing machines and can be easily used even by untrained operators.
Another important object of the present invention is to design a device which, while being inexpensive, consists of a small number of components and allows the needle travel to be varied in accordance with a plurality of positions and in accordance with parameters which can be fixed with the greatest degree of accuracy.